The only place where chroma key would help is a stationary flag getting partially obscured by a moving object. Otherwise, it’s more effective to just paste a drawing of all the hakenkreuze over each frame using decades-old subtitling technology. However, even stationary shots with non-moving flags (where the drawing can be reused) could be problematic because of picture jitter.

So what I meant was: the technique they would need was probably not chroma key.